Successful hip prosthetic surgery requires precise intra-operative placement and positioning of replacement structures as implants within the patient such that the in vivo function of the reconstructed joint is optimized biomechanically and biologically. For the surgeon, it is necessary to ensure that the replacement structural components are implanted correctly and function in situ properly in order to avoid intraoperative and post-operative complications, as well as to ensure a long-lasting action and use for the implanted prosthesis.
Important parameters for achieving a successful hip arthroplasty procedure include: (1) the position angles of the cup implanted in the patient's pelvis; (2) the position angle of the stem implanted in the patient's femur; and (3) the longitudinal placement of the stem.
A malpositioned hip prosthesis will not adequately restore the joint's biomechanics, will not function properly, and is at increased risk of intra-operative and post-operative complications. Such complications can include, without limitation, dislocation, impingement, fracture, implant failure, aseptic loosening, and subsidence. A malpositioned prosthetic implant is particularly susceptible to dislocation and early loosening because the prosthesis will not be well fitted or supported within the host's native bone.
One problem routinely faced by surgeons today concerning human hip replacement procedures is how to achieve proper acetabular prosthetic implant alignment. It is generally agreed among orthopedic surgeons that the ideal anatomic position (for most patients) for positioning the acetabular prosthetic implant within the native bone of the host's hip is at 45° (degrees) of inclination.
A second important angle is the angle of forward flexion, which ideally is at 20° (degrees). More recent advanced techniques emphasize “combined anteversion” of the reconstructed hip, rather than the cup's absolute angle of forward flexion. Combined anteversion is the sum of the angle of forward flexion of the cup plus the angle of anteversion of the stem. Since there is limited space for changing the stem's angle of anteversion, adjusting the position of the cup to that of the stem is critical to improving stability of the reconstructed hip and reducing impingement.
However, precise measurement of these specific angles, and therefore proper placement of the prostheses, has been difficult to achieve, mostly because two of these angles are relative to the patient's pelvis and the patient is covered by sterile surgical drapes during the course of the hip replacement operation. It also has not been possible to monitor any change in position of the patient's pelvis that can occur after draping the patient for the surgery.